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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Characterization of Geraniol Synthase from the Peltate Glands of Sweet Basil¹

Department of Molecular, Cellular and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109-1048 (Y.I., E.F., E.P.); Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721-0036 (D.R.G.); and Department of Vegetable Crops, Newe Ya'ar Research Center, Agricultural Research Organization, P.O. Box 1021, Ramat Yishay, 30095, Israel (E.L.)

The monoterpene fraction of the lemon-scented sweet basil (Ocimum basilicum) cv Sweet Dani consists mostly of citral (a mixture of geranial and neral), with lower levels of geraniol and nerol. These compounds are stored in the peltate glands found on the leaf epidermis. Younger leaves, which have a higher density of such glands, also have a higher content of monoterpenes than older leaves. Geraniol synthase (GES) activity, generating geraniol from geranyl diphosphate, was shown to be localized exclusively or almost exclusively to glands. GES activity resides in a homodimeric protein that was purified to near homogeneity. Basil GES requires Mn²⁺ as a divalent metal cofactor for activity and produces only geraniol from geranyl diphosphate. K_m values of 21 and 51 µM were obtained for geranyl diphosphate and Mn²⁺, respectively. In the presence of ¹⁸O-labeled water, GES catalyzed the formation of ¹⁸O-geraniol from geranyl diphosphate, indicating that the reaction mechanism of GES is similar to that of other monoterpene synthases and is different from the action of phosphatases. A GES cDNA was isolated based on analysis of a glandular trichome expressed sequence tag database, and the sequence of the protein encoded by this cDNA shows some similarity to sequences of other terpene synthases. The expression of the GES cDNA in Escherichia coli resulted in a protein with enzymatic activity essentially identical to that of plant-purified GES. RNA gel-blot analysis indicated that GES is expressed in glands but not in leaves of basil cv Sweet Dani, whose glands contain geraniol and citral, and not in glands or leaves of another basil variety that makes other monoterpenes but not geraniol or citral.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.032946.

^* Corresponding author; e-mail lelx{at}umich.edu; fax 734-647-0884.

Received September 4, 2003; returned for revision September 30, 2003; accepted October 8, 2003.

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